With recent remarkable developments in portable electronic equipments, communication equipments and the like, developments of secondary batteries with high density of energy are strongly demanded from the viewpoint of economic efficiency and reduction in size and weight of the equipment. Current secondary batteries with high density of energy include a nickel-cadmium battery, a nickel-hydrogen battery, a lithium-ion secondary battery, a polymer battery and the like. Among these batteries, the demand for the lithium-ion secondary battery is highly growing in a market of power source due to its dramatically enhanced life and capacity, compared with the nickel-cadmium battery or nickel-hydrogen battery.
Essentially differed from the principle of operation in a metal lithium battery as a primary battery, the lithium-ion secondary battery is based on the principle of operation such that lithium ions move forwards and backwards between a positive electrode and a negative electrode by charging and discharging. Therefore, in the lithium-ion secondary battery, shapes of positive electrode material and negative electrode material are unchanged in the course of by charging and discharging.
Meanwhile, a polymer battery is said to be smaller in energy density than the lithium-ion secondary battery. However, since the polymer battery can be made in a form of a sheet with 0.3 mm or less in thickness by using the same positive electrode, negative electrode and solid or gel electrolyte as those in the lithium-ion secondary battery, it is easy to make a package, so that a thinner type is highly expected to be produced. In view of such a characteristic of the polymer battery, it is increasingly demanded to develop a lithium-ion secondary battery enhanced in heat resistance and or liquid leakage resistance by using a polymer as an electrolyte.
Such a lithium-ion secondary battery includes, as shown in the after-mentioned FIG. 3, a positive electrode, a negative electrode, an electrolyte and a separator. Lithium cobaltate (LiCoO2) or manganese spinel (LiMn2O4) is primarily applied to the positive electrode of the lithium-ion secondary battery. A nonaqueous electrolytic solution mainly composed of an organic solvent, such as lithium perchlorate, is primarily applied to an electrolytic solution used as the electrolyte. The separator comprises a film which separates the positive electrode and the negative electrode from each other to prevent short-circuit between both the electrodes.
It is required for an active material used for the negative electrode of the lithium-ion secondary battery to have a large exploitable energy per unit weight or per unit volume to enhance the capacity of the lithium-ion secondary battery.
Negative electrode active materials for such a lithium-ion secondary battery, which have been proposed, include, for example, a composite oxide of lithium and boron in Patent Literature 1, a composite oxide of lithium and transition metal (V, Fe, Cr, Co, Ni, etc.) in Patent Literature 2, a chemical compound including at least one of Si, Ge and Sn, nitrogen and oxygen in Patent Literature 3, and an Si particle with a surface coated with a carbon layer by chemical vapor deposition in. Patent Literature 4.
Each of the negative electrode active materials proposed in Patent Literatures 1 to 4 can improve the charging-discharging capacity of the lithium-ion secondary battery to enhance the density of energy. However, in association with the charging and discharging of this lithium-ion secondary battery, the deterioration becomes eminent due to generation of dendrite and or a passive chemical compound on the electrode, or the expansion and contraction in occlusion and release of lithium ions significantly take place.
Therefore, this lithium-ion secondary battery is insufficient in the retention capability of discharging capacity for repeated charging and discharging (hereinafter referred also to as “cycle characteristic”). Further, required characteristics on lithium-ion secondary battery are not necessarily satisfied since the charging capacity does not achieve a large capacity exceeding 372 mAh/g in use of carbon as a negative electrode active material, and the initial efficiency that is represented by a ratio of discharging capacity to charging capacity (discharging capacity±charging capacity) of lithium-ion secondary battery just after mill production thereof is insufficient, and further improvements in the density of energy are desired.
To respond to such demands, it has been conventionally attempted to use a silicon oxide such as SiO as a negative electrode active material. The silicon oxide can be a negative electrode active material having a larger effective charging/discharging capacity, since it is low (less noble) in electrode potential to lithium and is inhibited from the deterioration such as collapse of crystal structure or generation of an irreversible substance due to occlusion and release of lithium ions during charging/discharging, and can reversibly occlude and release the lithium ions. Therefore, the silicon oxide can be expected, by using it as the negative electrode active material, to provide a secondary battery high in voltage and density of energy and also excellent in charging-discharging characteristic and cycle characteristic.
As efforts about the above-mentioned negative electrode material, for example, proposed in Patent Literature 5 is a non-aqueous electrolyte secondary battery using a silicon oxide, which allows occlusion and release of lithium ions, as the negative electrode material. This proposed silicon oxide contains lithium in its crystal structure or amorphous structure, and constitutes a composite oxide of lithium and silicon so that lithium ions can be occluded and released by electrochemical reaction in a non-aqueous electrolyte.
In the secondary battery proposed in Patent Literature 5, a high-capacity negative electrode active material can be obtained. However, according to the present inventors' studies, there is still room for further improvements toward the practical use since the irreversible part in capacity thereof in the initial charging/discharging is large, and the cycle characteristic is not sufficiently developed to a practical level.
In a lithium-ion secondary battery and a method for manufacturing the same proposed in Patent Literature 6, the negative electrode active material includes an oxide particle containing at least one element selected from a group consisting of Si, Sn, Ge, Al, Zn, Bi and Mg and a carbonaceous substance particle, said oxide particle being embedded in said carbonaceous substance particle.
In the manufacturing of the lithium-ion secondary battery proposed in Patent Literature 6, as described in embodiments thereof, a composite powder in which SiO particle is embedded in graphite particle, for example, by repetitively applying mechanical pressure bonding to amorphous SiO particle and natural graphite particle is used as a raw material to form an electrode by pressure molding for use as a negative electrode. Therefore, conductivity itself can be given to the pressure-molded negative electrode material. However, since the pressure molding is mechanical pressure bonding of one solid with the other, there arises a problem such that a uniform carbon film cannot be formed, so a uniform conductivity cannot be secured.
In Patent Literature 7, proposed as a negative electrode material for secondary battery is a composition prepared by mixing a silicon-based negative electrode active material including silicon or a silicon oxide doped with at least one element selected from those belonging to the 13th and 15th groups in the Periodic Table with a conductivity imparting agent, and as an example thereof, using silicon doped with boron as the negative electrode active material is described.
A negative electrode active material including simple silicon doped with boron, which is described as an example in Patent Literature 7, has an initial discharging capacity as large as 3000 mAh/g, and is improved in cycle characteristic and initial efficiency, compared with the one without boron-doping. However, the charging/discharging capacity is reduced to about 80% of the initial level after ten times of charging/discharging, and the cycle characteristic thus does not reach a practical level.
Examples with respect to silicon oxides are not described in Patent Literature 7.